Since the resource of earth is finite, and combustion will induce air pollution in the environment; the conventional way of deriving power from oil is replaced by electric power which is suitable for the requirement of environmental protection. For example, the electromotive vehicle is an apparent example. However, the power supplies are not popular in many places. A large charging time is required and the amount of power can not be seen. Therefore, a current detector is required for the driver because no one hopes to push an electromotive vehicle along a roadway due to exhaustion of power without a fault indication.
FIG. 4 shows a prior art electric quantity indicator, wherein in the initial position, a voltmeter is installed. The measuring result is indicated by a scale for viewing the storage electric quantity so as to estimate traveling length of the electromotive vehicle. As the user is at home, he may charge the battery in order to avoid a loss of power on a highway. However, such a design has some defects so that the real electric quantity can not be indicated. Since as a battery is stopped and rests for a period of time, the voltage will increase so as to present a virtual voltage as shown in FIG. 5. Thus, in the prior art, it is possible that the indicator shows that 90 percent of power is stored, but practically, only 30 percent of the power is stored. Therefore, a user will make a mistake due to an incorrect indication.